Solve for $r$ : $-15 + r = -8$
Solution: Add $15$ to both sides: $ \begin{eqnarray} \\ -15 + r &=& -8 \\ \\ {+15} && {+15} \end{eqnarray} $ $ \begin{eqnarray} \hphantom{-15 + r} &\hphantom{=}& \hphantom{-8} \\ r &=& -8 {+ 15} \\ \end{eqnarray} $ Simplify. $ r = 7$